This project is designed to evaluate the effects of aging on three prominent cell groups in the rat auditory pathway. These cell groups, the medial superior olivary nucleus (MSO), the ventral nucleus of the trapezoid body (VTB), and the medial nucleus of the trapezoid body (MTB), are components of the superior olivary complex which is known to function in the localization of sounds in space, and auditory behavior that has been shown to be impaired in agef rats. This project entails morphological studies of the MSO, VTB, and MTB in groups of animals at the following ages: 3 months (MO), 12 MO, 18 MO, 24 MO, and 30 MO, thus representative of the adult rat lifespan. The specific aims of the project are: 1) Analysis of calyx structure - Calyces of Held in the MTB's of an age-graded series of animals will be anterogradely labeled with horseradish peroxidase and analyzed light microscopically for aging changes in their branching patterns, 2) Analysis of neuron loss - Cell counts will be done on serial Nissl-stained transverse sections through the entire superior olivary complex in an age-graded series of animals. Since studies on the MTB have already been done, and the lateral superior olivary nucleus is currently being analyzed, this project will focus on the MSO and VTB, 3) Cell size determinations - The size of neurons in the aging MSO and VTB will be analyzed using quantitative light microscopy, to determine if cell shrinkage occurs in these cell groups with advancing age, and 4) Ultrastructural analysis - The fine structure of neurons, glial cells, and capillaries in the aging MSO and VTB will be studied using transmission electron microscopy. In addition, quantitative methods will be used to detect age-related loss of axon terminals synapsing with neurons in the MSO and VTB. Although the effects of aging on human and rat peripheral auditory structures have been described, relatively little research effort has been devoted to aging in central portions of the system. Thus, the role of central auditory changes in presbycusis is largely unknown. The proposed project should provide a better understanding of the extent to which central auditory structures are altered with advancing age, and the relationship of such changes to those occurring in other areas of the auditory system.